MFC r360071-r360072: force -fcommon for parts of kernel/kmods that need

Note that this isn't a 1:1 MFC, as a couple more instances needed to be
patched to force -fcommon in the face of a compiler that defaulted to
-fno-common.

r360071:
Allow kernel modules to build with a compiler that defaults to -fno-common

This uses the same approach as r359691.

r360072:
More fixes to build the kernel with a compiler that defaults to -fno-common

Using the same approach as the last commit for the files used by genassym.sh.

MFC: r351429, r351430, r351485, r351497, r351498

All changes related to disabling Werror on stable/11
to ease MFCs now that we've disabled them in head.

MFC r331056:
Share a single bsd-linux errno table across MD consumers

Three copies of the linuxulator linux_sysvec.c contained identical
BSD to Linux errno translation tables, and future work to support other
architectures will also use the same table. Move the table to a common
file to be used by all. Make it 'const int' to place it in .rodata.

(Some existing Linux architectures use MD errno values, but x86 and Arm
share the generic set.)

This change should introduce no functional change; a followup will add
missing errno values.

MFC r331057:
linux_errno.c: add newer errno values

Also introduce a static assert to ensure the list is kept up to date.

MFC r331060:
Chase r331057 in libsysdecode erno table

MFC r319052:

Strip _binary_linux_locore_o_size from ${VDSO}.so as it is a low absolute
symbol, and this breaks symbol lookup in ddb.

Approved by: re (kib)

MFC r314651,r318439,r318440:

r314651:

sys/modules: normalize .CURDIR-relative paths to SRCTOP

This simplifies make output/logic

Tested with: `cd sys/modules; make ALL_MODULES=` on amd64

r318439:

Normalize .PATH on SRCTOP

This will help Jenkins dedupe 9 warnings between the static build and
the module build of ipsec(4).

Missed in SRCTOP conversion in r314651.

MFC with: r314651

r318440:

Normalize SYSDIR on SRCTOP instead of .CURDIR

This is being done to simplify pathing for CFLAGS and source files.

MFC r314054: Exclude -flto when building *genassym.o

The build process generates *assym.h using nm from *genassym.o (which is
in turn created from *genassym.c).

When compiling with link-time optimization (LTO) using -flto, .o files
are LLVM bitcode, not ELF objects. This is not usable by genassym.sh,
so remove -flto from those ${CC} invocations.

Sponsored by: The FreeBSD Foundation

MFC r302517:

Fix a copy/paste bug introduced during X86_64 Linuxulator work.
FreeBSD support NX bit on X86_64 processors out of the box, for i386 emulation
use READ_IMPLIES_EXEC flag, introduced in r302515.

While here move common part of mmap() and mprotect() code to the files in compat/linux
to reduce code dupcliation between Linuxulator's

Approved by: re (gjb)